Cross posting this from ExPo so my apologies. I am getting ready to install a dual battery system, and in the planning/research stage. I am going to go with the Blue Sea 7622 as it will provide a good deal of functionality for the price, fairly straight forward installation, and I can leave my winch set up as-is. However, one thing this Blue Sea ACR doesn't do is charge the batteries separately. They are either isolated or in parallel.

Given this, I am wondering what my options are for the type of second (house) battery. I already have a Diehard platinum as the starting battery. A better type for the house battery would be a deep cycle, but do I need to worry about a constant charge/discharge between the two batteries when in parallel? Or will the higher charge voltage coming off the Alt. negate that?

Basically just wondering if it is a "better/more reliable" setup to go with two identical batteries and the downside of running a starting battery as a house battery that has the potential to be deeply discharged and thereby reducing its life? I won't be running a ton of stuff off the house battery; some cabin lights, fridge, etc. The fridge can be set to not draw down the battery as much, but having a deep cycle sure would be nice for the extra fridge time.

I suppose one downside to having a deep cycle would be if the starting battery was dead and needed to jump from the house battery, the deep cycle won't do as good a job in that aspect.

I run two identical AGM baterries in parallel. They both charge/discharge equally. Plenty of power for whatever. No extra wiring nonsense either, so you dont find out too late that your spare is dead or not being charged. Ive had mine that way for years with no problems. Its a good idea to run a voltage meter to keep an eye out on your battery health.

I did some addtional investigation and it seems people are speculating that the die hard auto and marine batteries are very similar, if not the same on the inside. cca/reserve capacity/Ah ratings all line up. Sears even lists the auto DHP battery as a deep cycle.

If thats the case then I'll just get another standard auto DHP in the same group size for both simplicity and the extra year warranty.

If they are not separated you should try to keep both of the same type and condition. Although, if they are only paralleled during active charging and are disconnected when the charging system is not running it would less of an issue.

I think over time you'll probably see one battery age quicker than the other. If I had to guess I think you'll ruin the deep cycle faster than the starter in this scenario, unless there's some sort of current limiting on it when they are brought into parallel. Deep cycles do not like a high current, short duration load and the in-rush of a depleted starting battery would want all the current it can get. In this case the second battery would react much faster than the alternator to fill it, thus the need for a current limit.

If they are not separated you should try to keep both of the same type and condition. Although, if they are only paralleled during active charging and are disconnected when the charging system is not running it would less of an issue.

I think over time you'll probably see one battery age quicker than the other. If I had to guess I think you'll ruin the deep cycle faster than the starter in this scenario, unless there's some sort of current limiting on it when they are brought into parallel. Deep cycles do not like a high current, short duration load and the in-rush of a depleted starting battery would want all the current it can get. In this case the second battery would react much faster than the alternator to fill it, thus the need for a current limit.

Thanks Dave. To the last point, I believe the blue sea module I was looking at has a start isolation function that will keep the batteries separated for 30-90 seconds after start, depending on voltage. I would imagine to somewhat mitigate that in-rush scenario you talk about.

Yes, I think it would work fine for most cases. That's also to give the charging system a chance to stabilize before parallel them. Only time I could imagine I'd be worried is when you've seriously exhausted your battery winching or left your headlights on or something. Even then once in a while is probably no biggie.

I forget to mention, just because two batteries are both AGM does not mean they are same. A starting battery and a deep cycle are physically constructed different, such that in some cases you will actually ruin a house battery trying to use it for a high current. The plates, the internal construction is not made to dump current in a hurry. Vice-versa, short, trickle loads cause starting batteries issues, they'll sulfinate faster, requiring maintenance charges and stuff to regain capacity.

But, OTOH, things like voltage will be fairly constant, so they can be charged in parallel as long as both are in good shape and there's sufficient current. Lead-acid batteries self regulate for the most part, so as their state of charge goes up, their internal resistance increases and current draw goes down. You would have more trouble potentially trying to do an AGM and flooded battery in parallel since they have slightly different voltage/current requirements.

Another thing with the deep cycle is you'll need to charge it periodically independent of the vehicle. A charger that's specific for AGM/deep cycles is best.. the alternator will keep the deep cycle topped off and will measure full charge, but it will drop quickly, faster than you think it should. Giving it a good trickle charge will slow that down though, something to do before a big trip, etc.

A few of us here have the C-tek chargers, if you search for that there's probably more battery theory about the differences. hth!